1. Field of the Invention
This invention relates to sensing single and multiple bit non-volatile memories to determine what value is represented. In particular, it relates to a device and method for sensing the source and drain of the cell being read.
2. Description of the Related Art
Sense amplifiers are used to determine the data value or values stored in a non-volatile memory. In a conventional sensing scheme, a current through the memory cell being sensed is compared to a reference current by a current-sensing sense amplifier. The ratio of the desired cell current to the reference current is referred to as the sensing ratio SR. A conventionally preferred sensing ratio is 2:1. When the current through the selected cell is more than twice the reference current (Ir), a value of "1" is sensed. When the current is significantly less than twice the reference current, a value of "0" is sensed. A current close to the sensing ratio may take longer to sense or may produce an indeterminate result. Manufacturing and operating variations limit the effectiveness of current sensing.
Newer, low voltage memory cells and multi-bit memory cells are difficult to sense using the conventional current sensing scheme described above. Low voltage memory cells are designed to minimize current flow. Multi-bit memory cells have less read margin, due to the multiple threshold voltages that define the multiple bit values of a cell. Accordingly, a novel approach that relies on sensing both the source and drain of a selected cell, as an alternative to sensing only the drain, is described below.